Microbiota, Dysbiosis, and Mucosal Immune Homeostasis

The mucosal immune system develops and operates in constant dialogue with the trillions of microbes that colonize the gut and other barrier surfaces. The resident microbiota help instruct immune development, calibrate immune responses, and maintain homeostasis, while the immune system in turn shapes the microbial community; disruption of this balance, called dysbiosis, is linked to immune-mediated disease.

Definition

The microbiota is the community of microorganisms colonizing mucosal surfaces; in immunology it denotes the commensal microbes that co-regulate mucosal immune development and homeostasis, whose disturbed composition or function is termed dysbiosis.

Scope

This topic covers how commensal microbes and their metabolites shape mucosal immunity, the concept of host-microbiota homeostasis, and the notion of dysbiosis as a disturbed microbial-immune balance. It is a reference entry in mucosal immunology and is not clinical guidance.

Core questions

• How does the microbiota shape the development and function of mucosal immunity?
• How do microbial metabolites influence immune cells at barrier surfaces?
• How does the immune system maintain homeostasis with commensal microbes?
• What is dysbiosis and how is it related to immune-mediated disease?

Key concepts

• Commensal microbiota
• Host-microbiota homeostasis
• Short-chain fatty acids
• Regulatory T cell induction
• Colonization resistance
• Dysbiosis
• Immune instruction by microbes

Mechanisms

Commensal microbes and their products are sensed by the mucosal immune system and help drive immune maturation, including the induction of regulatory and effector T-cell populations and the production of IgA. Microbial metabolites, particularly short-chain fatty acids generated by fermentation of dietary fibre, act on immune and epithelial cells; for example, they promote colonic regulatory T cell homeostasis. The microbiota also provides colonization resistance against pathogens, while the immune system, through secretory IgA and antimicrobial peptides, constrains and organizes the microbial community. This reciprocal regulation maintains homeostasis, and its disturbance, dysbiosis, is associated with inflammatory and immune-mediated conditions, though causal direction is often complex.

Clinical relevance

Microbiota-immune interactions are implicated in inflammatory bowel disease, allergy, and other immune-mediated conditions, and they inform interest in microbiome-targeted approaches; this is an area of active research rather than settled clinical practice. The entry describes mechanisms and associations for reference and is not a basis for individual diagnosis or treatment.

History

Once germ-free animal models showed that the immune system fails to develop normally without microbes, the microbiota moved to the centre of mucosal immunology. Subsequent work defined specific microbial influences on T-cell populations, identified metabolites such as short-chain fatty acids as mediators, and framed dysbiosis as a disturbance of the microbial-immune balance relevant to disease.

Debates

Is dysbiosis a cause or a consequence of immune-mediated disease?

Altered microbial communities are consistently associated with inflammatory and immune-mediated conditions, but distinguishing whether dysbiosis drives disease or results from the altered immune and inflammatory environment remains difficult and is an active question.

Key figures

• Yasmine Belkaid
• Lora Hooper
• Dan Littman
• Kenya Honda
• Wendy Garrett

Seminal works

• hooper-2012
• belkaid-2014
• smith-2013
• honda-2016

Frequently asked questions

How does the microbiota influence the immune system?

Commensal microbes and their metabolites instruct the development and calibration of mucosal immunity, for instance by promoting regulatory and effector T cells and IgA production and by providing colonization resistance against pathogens.

What is dysbiosis?

Dysbiosis is a disturbance in the composition or function of the microbiota relative to a healthy balance; it is associated with immune-mediated and inflammatory diseases, although whether it is a cause or consequence often remains unclear.